This is a hard question that is open to discussion even among historians who specifically study World War 2 as a topic and specialize in it.
The most frequent answers that would probably be given is: 1. Pearl Harbor - with the attack of the Japanese on Pearl Harbor, USA was effectively dragged into the war which may have tipped the odds in favour of the Allies. 2. Stalingrad - the Soviet Union captured a huge German army in Stalingrad. The soldiers from this army were either imprisoned until the rest of the war or died due to hunger. In effect this meant that the German forces lost a whole army on the Eastern front. 3. Normandy landings - the Normandy invasion gave the Allies a foothold on the beaches of Western Europe from where they could invade through the mainland right into Berlin by the end.
The Constitution was drafted in 1787, after the American Revolution ended. What document preceded the Constitution in outlining the powers of the US government? the Declaration of Independence.
